1. Field of the Invention
This invention relates in general to resistive well logging devices and more particularly to resistive well logging devices which permit measurement of formation resistivity at multiple radial depths from the borehole.
2. Description of the Prior Art
In known drilling operations, the drilling fluid or "mud" in the borehole is conditioned such that the hydrostatic pressure of the mud column is greater than the fluid pressure of the formation. The differential pressure between the hydrostatic pressure of the mud column and the fluid pressure of the formation forces mud filtrate into the permeable formations which are traversed by the borehole, whereby solid particles of the drilling mud are deposited on the borehole wall to form a layer known as "mudcake". This mudcake usually has very low permeability and considerably reduces the rate of infiltration as the mudcake thickens. In the area of the formation which immediately surrounds the borehole all of the formation water and some of the hydrocarbons, if present, are flushed away by the mud filtrate. This area is referred to in the art as the "flushed zone". The resistivity of the so-called flushed zone is generally referred to as R.sub.XO. Radially displaced even farther from the borehole is an area where the displacement of formation fluids is less and less complete, resulting in a transition zone in which a progressive change of resistivity occurs from the resistivity of the flushed zone to the resistivity of the uninvaded formation. The resistivity of the uninvaded formation is generally referred to as R.sub.t. The lateral depth of invasion by mud filtrate depends in part on formation permeability and is quite variable, ranging from less than one centimeter to several tens of centimeters.
Measurements of the resistivity of the flushed zone are important for several reasons. In those formations where invasion is moderate to deep, knowledge of the flushed zone resistivity value makes possible more accurate determinations of the true resistivity of the formation, which can be utilized to determine the likelihood of hydrocarbon saturation. Also, some methods for computing hydrocarbons saturation are determined by utilizing a ratio of the resistivity of the flushed zone to the true formation resistivity. More recently, the resistivity of the flushed zone may be utilized in conjunction with a full evaluation of hydrocarbon effects on both neutron and density logs. Moreover, the comparison of the resistivity of the flushed zone and the true resistivity are useful for understanding the migration of hydrocarbons in the formation.
In addition to recognizing the usefulness of the resistivity of the flushed zone, the prior art has been concerned with acquiring information which is indicative .of the location of lateral discontinuities in electrical resistivities of the borehole surrounding materials in order to determine well borehole diameter and depth of fluid invasion. For example, U.S. Pat. No. 2,754,475 discloses that resistivity measurements may be made at various lateral depths in the borehole and formations surrounding the borehole by measuring throughout a plurality of borehole locations the average resistivity of formation and borehole fluid contained within respective spherical shells. These shells are established by an electrical field which converges substantially radially through surrounding formations upon an input electrode in the fluid within the well borehole. The borehole radius, depth of invasion, and the location of the uninvaded formation can be determined by identifying any discontinuities which appear in a display.
Still more recently it has been determined that a wall-engaging pad device is the most appropriate method for the measurement of shallow invasion. Many such devices have been proposed. U.S. Pat. No. 2,669,688 discloses a wall-engaging pad device which includes a three electrode system comprising a current electrode and two potential measuring electrodes, with the current return being located on the cable. Measurements made with this system are indicative of the resistivities at different shallow lateral depths of investigation. One depth of investigation is approximately equal to the probable thickness of mudcake on the wall of the borehole, and the other is slightly greater so as to include the mudcake and at least a portion of the adjacent portion of the formation that has been invaded by the mud filtrate. Since the presence of mudcake on the wall of a borehole is an indication of invasion of the formation by mud filtrate, proper interpretation of the measurement so made enables permeable formations to be identified. A third measurement at yet another shallow lateral depth of investigation also facilitates the identification of permeable formations.
In order to accurately measure the value of the resistivity of the flushed zone, the measurement must not be affected by the borehole or, in the alternative, must be capable of being corrected for borehole effects. Measurements made at different shallow lateral depths of investigation can be corrected when at least one of the measurements accurately yields mudcake resistivity. Borehole effects on the resistivity of the flushed zone measurement may be minimized by the utilization of focussing currents to accurately control the path taken by the survey current. Borehole effects, particularly severe as mudcake thickness increases, arise when the survey current is shunted back to the borehole by the relatively low resistance path formed by the mudcake, so that the formation measurement is influenced to a large extent by the mudcake resistivity. Several prior art devices have been proposed to overcome this problem. Generally, these types tools utilize a focussed pad or wall-engaging member to determine the lateral thickness of a mudcake as well as the resistivity of the invasion zone.
Wall-engaging pad devices have also found application in the determination of dip. Dip determining devices, known in the prior art as dip meters, typically employ four pads which are applied against the borehole wall at spaced locations. Typically, each pad includes a transducer which conducts an investigation of formation characteristics immediately adjacent to that pad. An individual pad may be provided with more than one electrode for the purpose of improving the detection of bed boundaries.
Dip meter tools typically use passively focussed electrode systems for constraining the surveying current to penetrate laterally for an appreciable distance into the adjacent earth formation. Typically, the focussing current electrode is a metallic surface which forms the major portion of the pad face. Centrally located in the pad face is a recess covered by a layer of insulating material. A survey current electrode is disposed in the recess and is separated from the metal pad proper by an insulating material. Survey current emitted from the survey electrode is caused to penetrate laterally into the adjacent earth formation by the focussing current emitted from the focussing electrode. Additional focussing current may be emitted from the conductive surface of the dip meter sonde body. Where more than one survey electrode is provided, the additional electrodes are also completely surrounded by the focussing electrodes such that the respective survey current beams are focussed as described above by current emitting from the focussing electrode. The current return is generally provided via a return electrode which is located on the lower end of a multi-conductor cable, on the cable bridle, or on a tool body member.
It should therefore be obvious that a need exists for an improved apparatus for measuring the resistivity of a subsurface formation traversed by a borehole to a series of lateral depths of investigation.